Patent GB-1227335-A (GB)
IPC: B60T8/94
Filed: 1967-07-15
Abstract: 1,227,335. Servo-assisted braking systems; skid sensing. HARRY FERGUSON RESEARCH Ltd. 9 July, 1968 [15 July, 1967], No. 32644/67. Heading F2F. A servo-assisted braking system includes a servo-mechanism with an energizing source, a sensing device adapted to signal in response to inertia change, valve means connecting the energizing source to the servo-mechanism and operable to vary brake-assisting action thereof in response to a signal from the sensing device, and a fail-safe valve providing a normally-open connection between the valve means and the servo-mechanism, and a normally-closed connection between the energizing source and the servo-mechanism, and operable to close the connection between the valve means and the servo-mechanism and simultaneously to open the connection between the energizing source and the servo-mechanism automatically after a predetermined time lapse from operation of the valve means. As shown, a vacuum servo-mechanism 10 cooperating with an hydraulic tandem master cylinder 15, a vacuum tank 11, an inertiasensing device 12 and a change-over valve 13 are constructed and operate somewhat as described in Specification 1,101,582. Thus, during normal braking, vacuum is applied to the left hand side of diaphragm 16 in the servo-mechanism 10 by tank 11, pipe 18, valve 13, and pipes 38 and 21, as shown. When the inertia-sensing device 12 is operated (e.g. by excessive deceleration of the vehicle transmission), the valve members in the change-over valve 13 are moved to the left from the position shown, whereupon vacuum is applied to the right of diaphragm 16, to oppose braking action, by tank 11, pipe 18, valve 13, and pies 23 and 25. According to the invention, a further valvethe "fail-safe" valve 14-is interposed between the vacuum supply pipe 18 and the vacuum delivery pipes 38, 21 (these last two being considered as one pipe from the point of view of Specification 1,101,582). The function of this valve is to ensure that if a fault occurs in the sensing device 12 and/or in the change-over valve 13, so that the latter remains in its changedover state for a relatively long period, a connection will be opened between the vacuum tank and the left-hand side of diaphragm 16 to reestablish normal servo-assisted braking conditions. Operation.-In the normal condition of the fail-safe valve 14, which is as shown, atmospheric pressure is admitted to the lower side of a diaphragm 35 by a permanent passage 41, while vacuum is admitted to the upper side by pipe 18, ports 19, 20 and pipe 39 (containing restricting orifice 40). Valve element 33 is thereby held in its upper position, where bypass 18A, 31, 30, 21 is cut off, and passage 38, 29, 30, 21 is maintained open. When the valve 13 has been changed-over, and remains so for a sufficient length of time, the restricting orifice 40 permits atmospheric pressure (which now prevails in pipe 38 by reason of air inlet 43 in the lower part of valve casing 13) to become established in sub-chamber 28A above diaphragm 35. Spring 32 now lowers valve element 33 to open the by-pass and close the passage 38, 29, 21. Modification.-The fail-safe valve may be incorporated in vacuum servo-assisted braking systems where the change-over valve operates only to reduce or remove the supply of vacuum to a single-acting servo-mechanism. The servosystem may be pressureinstead of vacuumoperated, in which case, during normal operation, the energizing source 11 would be connected to the under-side of diaphragm 35 and the air passage 41 to the upper side. The sensing device 12 may include a ball-andramp device (not illustrated or described). A manually operable valve may be included in pipe 39 to open it to atmosphere, thus bypassing valve 13, should the anti-skid effect not be required, e.g. in a sporting event. The fail-safe valve may form a sub-assembly on the change-over valve and/or the diaphragm may be replaced by a piston. A toggle may be included in the connecting means to provide snap-action during movement of the valve element.